Characteristics of Near-Inertial Oscillations in the East Sea using High-Resolution Surface Drifters at Hourly Intervals

초록

Near-inertial oscillations (NIOs) in the ocean’s surface layer are primarily generated by wind, with a portion of the energy transferred below the mixed layer, contributing to vertical mixing. Previous studies on NIOs in the East Sea have been conducted using fixed-point Eulerian moorings or short-term Lagrangian observations. Recently, numerical models driven by high-resolution wind stress forcing with a temporal resolution finer than three hours have enabled detailed discussions of NIO characteristics over the East Sea. However, the temporal and spatial characteristics of NIOs covering the entire East Sea have not been thoroughly investigated based on observational data. This study analyzes 27 years of high-resolution Global Drifter Program (GDP) surface drifter data at hourly intervals and revisits the temporal and spatial characteristics of NIOs over the whole East Sea, alongside the reanalysis of data on wind and mixed layer depth. The average and median magnitudes of NIOs observed from the surface drifters over the entire period were 0.09 m/s and 0.06 m/s, respectively. Wind energy input, indicating wind-current resonance, and NIOs were stronger from July to December. Moreover, strong NIOs were observed across all seasons at locations where significant wind energy input was described. Notably, NIOs exceeding 0.7 m/s were observed primarily from July to September. The climatological monthly maximum of NIOs occurred in September, which was attributed to the strongest wind energy input (> 80 mW m-2), including the influence of typhoons, as well as shallow mixed layer depths (September mean of 18m). The decay timescale of NIOs was found to be approximately 4 days, with significantly longer timescales in the north of 39°N compared to the southern regions, as suggested in previous studies. © 2024 Ocean and Polar Research.

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